The easiest way to attack a country with a nuclear weapon is by shipping the weapon to the target nation as cargo. A rogue state or terrorist organization that manages to acquire a nuclear weapon could pack it into a maritime cargo container, railcar, truck, or even a large van. Once inside the target nation, the weapon could be sequestered in an urban environment, a government center, or adjacent to a military base for unlimited extortion or a devastating attack at any moment.
Nuclear weapons are hard to detect. They are radioactive but not highly so, and standard shielding can reduce the radiation signature further. The primary signature of a nuclear weapon is MeV-range neutrons and gamma rays, along with low-energy neutrons moderated by surrounding materials. The main inspection challenge is to detect these particles in the presence of natural backgrounds and benign radioactive substances that are commonly present in cargo and in the environment. Backgrounds also include cosmic rays (principally muons and electrons), gamma rays from radioactive materials in the soil, plus occasional low-energy neutrons generated by cosmic ray interactions in the atmosphere.
A major advantage would be a gamma ray or neutron detector that indicates where the radiation is coming from. The directional information would greatly enhance the statistical power of each detection, since a small number of particles all coming from a specific location would trigger a secondary inspection immediately.
What is needed, then, is an efficient detector of gamma rays or neutrons or both, with high sensitivity in the energy ranges expected for clandestine weapons, and capable of determining the location of the source, rapidly and without searching or iteration. Such a detector should have sufficient sensitivity to localize even a well-shielded nuclear weapon among clutter and obfuscation, automatically, and at low cost. Such a detector would greatly enhance security against clandestine nuclear and radiological weapons, and would also speed up legitimate border traffic by passing clean loads quickly. And, when a secondary inspection is triggered, the inspectors could use the indicated source location as a starting point.